MSK - Biochemistry - Actin & Intermediate Filaments; Collagen; ECM Flashcards
What is the inheritance pattern of Duchenne’s muscular dystrophy?
What is the life expectancy of affected individuals?
X-linked recessive;
< 35
Duchenne’s muscular dystrophy affects which muscles first?
What muscles often appear hypertrophied?
What is the usual cause of death?
The pelvic girdle;
calf muscles;
loss of function of diaphragmatic and cardiac muscle
What type of muscle is the principal type affected in Duchenne’s muscular dystrophy?
What is a common first sign?
Voluntary;
failure of child to walk normally or hold normal posture - may walk on toes due to muscle contractures
Dystrophin binds actin to ____________, which binds to extracellular laminin.
α,β-dystroglycan
What disease is similar to Duchenne’s muscular dystrophy but is less severe and results later in life?
Becker muscular dystrophy
What two substances (one mineral and one nucleotide) are vital to F-actin polymerization from G-actin?
ATP, Mg2+
The ATP-binding cleft is on which end of the G-actin monomer?
The - end
G-actin must be bound to ___ to polymerize into F-actin.
α- and β-tubulin must be bound to ___ to polymerize into microtubule protofilaments.
ATP;
GTP
True/False.
Both the GTP associated with microtubules and the ATP associated with actin will hydrolyze relatively soon after polymerization?
True
(although the ATP-actin hydrolyzes much sooner)
What are the three cellular proteins regulating actin polymerization?
Cofilin,
profilin,
thymosin-β4
What protein breaks off pieces of F-actin?
Cofilin
What protein exchanges ADP for ATP on G-actin, allowing it to be used in polymerization?
(It just adds a phosphate group to the ADP)
Profilin
What protein sequesters ATP G-actin to prevent polymerization?
Thymosin-β4
What protein binds the + end of F-actin to prevent polymerization?
What protein binds the - end of F-actin to stabilize actin?
CapZ;
tropomodulin
What protein binds the + end of F-actin to prevent polymerization?
CapZ
What protein binds the - end of F-actin to stabilize actin?
Tropomodulin
What protein helps promote the unbranched form of F-actin?
What protein helps promote the branched form of F-actin?
Formin;
Arp2/3
What type of filament is especially important in cell migration (e.g. chemotaxis)?
Actin (microfilaments)
What are the four steps of cell movement during chemotaxis?
1. Foot process (lamellipodium) formation
2. Adhesion at the front of the cell
3. Translocation as the cell shifts forwards
4. Deadhesion at the back of the cell
What type of protein cross-links the cytoskeleton to the ECM?
Integrins
What type of protein cross-links actin into a meshwork on the P-face?
Spectrin
Hereditary spherocytosis can be caused by defects in what three proteins?
Spectrin, protein 4.1, ankyrin.
What type(s) of intermediate filament are especially prevalent in epithelial tissues?
Cytokeratins (acidic and basic)
What type(s) of intermediate filament are especially prevalent in muscle?
Desmin
What type(s) of intermediate filament are especially prevalent in mesenchymal tissues?
Vimentin
What type(s) of intermediate filament are especially prevalent in glial cells?
GFAP
What type(s) of intermediate filament are especially prevalent in neural tissues?
Neurofilaments
What type(s) of intermediate filament is especially prevalent in the nucleus?
Lamins
What protein other than spectrin or dytrophin is especially important in cross-linking actin into its structure on the inner cell membrane?
Fimbrin
What protein is especially important in organizing actin stress fibers during cell migration?
α-actinin
Spectrin and dystrophin are both in what supergene family?
Spectrin
What are some of the physical features commonly seen in type I osteogenesis imperfecta?
What are some other common occurrences?
Blue sclera, triangular shape;
frequent fractures, hearing loss (beginning in the twenties), scoliosis, thin skin, loose joints, low muscle tone, brittle teeth
What inheritance does type I osteogenesis imperfecta exhibit?
It is due to a defect in what?
Autosomal dominant;
type I collagen
What type of osteogenesis imperfecta frequently results in death during embryogenesis or birth?
Type II
What gene is associated with osteogenesis imperfecta type I?
COL1A1
How is osteogenesis imperfecta treated?
What is the mechanism of action?
Bisphosphonates;
osteoclast inhibition
_______________ are useful for treating osteoporosis and osteogenesis imperfecta by inhibiting osteoclasts because they mimic ______________, which is needed for normal osteoclastic activity.
Bisphosphonates;
pyrophosphates
How do individuals with type II osteogenesis imperfecta typically die?
Severe bone fracturing during the birthing process
What effect do bisphosphonates tend to have on apoptosis in bone cells?
Increased osteoclast apoptosis;
decreased osteoblast apoptosis
The basic structure of collagen that gives it its strength is a:
triple helix
What is the basic repeating amino acid code for collagen?
What amino acids make up the variable portions?
Gly - X - Y;
X,Y = proline or hydroxyproline
Is the collagen helix a right- or left-handed helix?
Left
What type of bonding is extremely prevalent in collagen, binding the strands together?
Hydrogen bonding
Describe collagen in terms of tensile strength and elasticity.
Describe elastin in terms of tensile strength and elasticity.
High tensile strength, low elasticity;
low tensile strength, high elasticity;
Vitamin C is needed for what step of collagen formation?
Why is this step important?
Proline hydroxylation;
it stabilizes the triple helix
True/False.
Without Vitamin C, collagen cannot be synthesized.
False.
Vitamin C is responsible for post-translational proline hydroxylation.
(without vitamin C, the collagen strands are simply less stable and more easily degraded)
Vitamin C is a cofactor of what enzyme involved in post-translational collagen modification?
4-prolyl-hydroxylase
Which five collagen fibrils follow the characteristic triple helix structure?
I, II, III, V, XI
What type of collagen is a higher order structure found in the basement membrane?
Type IV
What type of collagen is an anchoring type that connects the dermal and epidermal layers?
Type VII
What organ systems are most susceptible in Alport’s syndrome?
What is the main genetic defect in Alport’s syndrome?
Kidney, ears (ossicles), and eyes (cornea);
type IV collagen defects
What is the most common form of inheritance seen in Alport;s syndrome?
X-linked
Where is type I collagen found?
Most connective tissue
Where is type II collagen found?
Cartilage and vitreous humor
Where is type III collagen found?
Extensible connective tissue
Where is type IV collagen found?
What structure does it have?
Basal lamina;
sheet-like networks
Where are type V and XII collagen found?
Tissues containing type I collagen
Where is type VII collagen found?
Anchoring basal lamina to underlying connective tissue
What protein serves as a scaffold for tropoelastin?
Mutations in this protein lead to what disorder?
Fibrillin;
Marfan’s syndrome
What is a (typically present) symptom of homocystinuria that differentiates it from Marfan’s syndrome?
Intellectual disability
What are the first three steps of collagen synthesis?
(starting with a transcribed collagen mRNA)
1. mRNA is translated into pre-procollagen
2. Hydroxylation of select proline residues
3. Glycosylation of select lysine residues
What are the second three steps of collagen synthesis?
(starting here with hydroxylated and glycosylated procollagen chains)
- Three pro-alpha-chains come together into an alpha-helix
- Procollagen secreted from cell
- Procollagen ends cleaved off by procollagen peptidase
(This is now tropocollagen)
Following secretion of procollagen from the cell and cleavage of its ends to form tropocollagen, what happens?
Lysyl oxidase cross-links mature collagen fibrils
A deficiency of lysyl oxidase leads to what disorder?
Lysyl oxidase is used to cross-link what together?
What element is necessary for its function?
Menke’s syndrome;
collagen fibrils;
copper
Ehrlos-Danlos syndrome is caused by a mutation in the ADAMTS2 gene. This leads to a deficiency in what enzyme?
What does this enzyme do?
Procollagen peptidase;
cleaves procollagen ends to form tropocollagen
What are three of the few enzymes in the body that require copper (Cu2+) as a cofactor?
Tyrosinase,
dopamine β-hydroxylase,
lysyl oxidase
How does the ECM serve as a source of growth and chemotactic factors?
Growth factors are bound to (sequestered in) proteoglycans until metalloproteases release them
What is the basic makeup of ECM?
(3 main players)
Hydrated gel
+
reinforcing fibers throughout
+
multiadhesive signaling proteins
What proteoglycan is associated with the basement membrane?
What collagen type is associated with the basement membrane?
What multiadhesive proteins are associated with the basement membrane?
Perlecan;
type IV;
fibronectin, laminin
What proteoglycan is associated with the basement membrane?
Perlecan
What collagen type is associated with the basement membrane?
Type IV
What multiadhesive proteins are associated with the basement membrane?
Fibronectin, laminin
What are the two main functions of multiadhesive proteins in the ECM?
Signaling,
binding cells in place within the ECM
What is a glycosaminoglycan (GAG) in terms of structure?
Are they osmotically active?
An unbranched polysaccharide made of repeating disaccharide units (often a uronic acid + an amino sugar);
yes, they keep tissues hydrated by storing water
Are glycosaminoglycans eosinophilic or basophilic?
Why?
Basophilic;
lots of SO4- and COO- groups
What is a proteoglycan (found in the ECM)?
A core protein with tons of attached glycosaminoglycans
(basically a protein with tons of sugars sticking off of it)
(one branch of the ‘Christmas tree’ seen in the attached illustration)
What is the major core protein of proteoglycans found on articular cartilage?
True/false.
It can absorb and release synovial fluid as needed (according to pressures).
Aggrecan;
true
What are the two main cartilaginous proteins that are often degraded in osteoarthritis?
Type II collagen;
aggrecan
Is hyaluronic acid a proteoglycan?
No;
it is a glycosaminoglycan forming the ‘trunk’ to which proteoglycan ‘branches’ attach
What type of proteoglycan is primarily found in cartilage?
What type of proteoglycan is primarily found in basal lamina?
What type of proteoglycan binds type I collagen and is widespread in ECM?
Aggrecan;
perlecan;
decorin
What two proteoglycans and one protein sequester TGF-β?
Aggrecan, decorin
(storing it until the tissue is damaged and growth is needed);
fibrillin
(that’s why Marfan’s syndrome patients have such long bones)
Why do individuals with Marfan’s syndrome grow such long bones?
Fibrillin (which is missing or defective in these patients) sequesters TGF-β
What is a super-abundant protein of the ECM that binds type I collagen, fibrin, and proteoglycans to integrins on cell membranes, essentially binding cells to ECM?
Fibronectin
What is fibronectin’s main function?
Connecting cells to ECM
Multiadhesive proteins are ECM proteins such as fibronectin and laminin. What is another protein in this family that is essential to proper blood coagulation?
von Willebrand’s factor
What is the principal multiadhesive protein of the basal lamina?
What cell membrane protein does it bind?
What particular basal lamina protein does it bind?
Laminin-1;
α,β-dystroglycan;
type IV collagen
What are the two main families of protein involved in cell-to-cell adhesion?
Cadherins (found in desmosomes);
CAMs (cell adhesion molecules)
Inflammatory molecules induce endothelial cells to express what protein that loosely interacts with leukocytes?
After that initial velcro-like contact, what stronger protein binds the leukocyte and helps is extravasate?
P-selectins;
integrins
Describe integrin structure.
True/False.
Integrins are present in both active and inactive forms.
Heterodimeric (α and β)
True.
The inactive conformation of an integrin protein is:
The active conformation of an integrin protein is:
Bent;
extended
What are a few intracellular pathways that may be involved in different integrin protein signaling?
ERK 1/2
The Phosphatidyl Inositol 3- Kinase (PI3K) pathway
Rac / Rho / Cdc42 G-proteins
The _ _ _ sequence is important to integrin signaling.
RGD
What multiadhesive protein is often found in the basal lamina along with perlecan?
Agrin
Are collagen chains stabilized by interchain or intrachain hydrogen bonds?
Interchain
(there are no intrachain H-bonds)
